Preheat your oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing it or lining it with parchment paper.
In a large mixing bowl, combine the melted butter, granulated sugar, and packed brown sugar. Whisk until smooth. Add the eggs one at a time, mixing thoroughly after each. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually mix this into the wet ingredients until just combined.
Pour the brownie batter into the prepared pan and spread evenly. Bake for 25-30 minutes, testing for doneness with a toothpick. Allow to cool completely in the pan.
In a medium bowl, blend the peanut butter and powdered sugar until smooth. Spread this mixture over the cooled brownies.
In a microwave-safe bowl, combine the chocolate chips and vegetable oil. Microwave in 30-second intervals, stirring until melted. Pour over the peanut butter layer and spread evenly.
Refrigerate the brownies for about 30 minutes to set the chocolate topping.
Remove from the pan using the parchment paper and cut into squares for serving.
Notes
Garnish with flaky sea salt or crushed peanuts for added texture.